Charge-Transfer Salts of 6,6-Dicyanopentafulvenes: From Topology to Charge Separation in Solution.

6,6-Dicyanopentafulvene derivatives and metallocenes with redox potentials appropriate for forming their radical anions form highly persistent donor-acceptor salts. The charge-transfer salts of 2,3,4,5-tetraphenyl-6,6-dicyanofulvene with cobaltocene (1⋅Cp2 Co) and 2,3,4,5-tetrakis(triisopropylsilyl)-6,6-dicyanofulvene with decamethylferrocene (2⋅Fc*) have been prepared. The X-ray structures of the two salts, formed as black plates, were obtained and are discussed herein. Compared with neutral dicyanopentafulvenes, the chromophores in the metallocene salts show substantial changes in bond lengths and torsional angles in the solid state. EPR, NMR, and optical spectroscopy, as well as superconducting quantum interference device (SQUID) measurements, reveal that charge-separation in the crystalline states and in frozen and fluid solutions depends on subtle differences of redox potentials, geometry, and on ion pairing. Whereas 1⋅Cp2 Co reveals paramagnetic character in the crystalline state and in solution, compound 2⋅Fc* shows a delicate balance between para- and diamagnetism, depending on the temperature and solvent characteristics.

The 6,6-dicyanopentafulvene core: a template for the design of electron-acceptor compounds.

The electron-accepting ability of 6,6-dicyanopentafulvenes (DCFs) can be varied extensively through substitution on the five-membered ring. The reduction potentials for a set of 2,3,4,5-tetraphenyl-substituted DCFs, with varying substituents at the para-position of the phenyl rings, strongly correlate with their Hammett σp-parameters. By combining cyclic voltammetry with DFT calculations ((U)B3LYP/6-311+G(d)), using the conductor-like polarizable continuum model (CPCM) for implicit solvation, the absolute reduction potentials of a set of twenty DCFs were reproduced with a mean absolute deviation of 0.10 eV and a maximum deviation of 0.19 eV. Our experimentally investigated DCFs have reduction potentials within 3.67-4.41 eV, however, the computations reveal that DCFs with experimental reduction potentials as high as 5.3 eV could be achieved, higher than that of F4-TCNQ (5.02 eV). Thus, the DCF core is a template that allows variation in the reduction potentials by about 1.6 eV.